The present invention relates to a beam deflection technique for simultaneous measurements of the thickness, refractive index and optical absorption of transparent materials using a charge coupled device (CCD) camera. The method and apparatus is particularly suited to measuring flat, thin materials.
Many optical devices such as spectrophotometers use absorption spectroscopy to measure the concentration of various materials. There are two primary procedures used for measuring concentration. An absorption value of a material is calculated at an absorbing wavelength and the absorption of the material at a minimally absorbing wavelength is subtracted. This process of xe2x80x9cblankingxe2x80x9d minimizes uncertainties due to sample cell imperfections and parasitic scattering. Another procedure involves taking absorption measurements on a material at various time intervals and analyzing the differences between measurements. In these methods, as well as in other technologies such as scattering techniques and optical cavity based techniques, the accurate knowledge of the refractive index of the material and optical path length is critical information.
In these methods of concentration measurement, the reflectivity losses due to the index of refraction are generally assumed to be a function only of wavelength. This assumption introduces uncertainties in absorption measurements because the refractive index also directly depends on the number density and the type of polarizable species in the material. As a result, there is a need to develop techniques to measure accurately refractive indices of samples at the specific optical absorption wavelength.
Accurate thickness (optical path length) data for a given material greatly enhances the sensitivity and accuracy of spectrophotometers and other absorption devices in concentration measurement. Furthermore, the need to accurately monitor and control the refractive index and thickness of samples during production exists in the manufacturing of materials such as laboratory windows, optical lenses, automotive parts, and optical glasses.
The refractive index and optical path are measured by a method described in U.S. Pat. No. 6,057,928, issued to Li et al. This technique measures the variations in reflectance (ratio of the reflected beam over the incident beam powers) and the beam phase distribution as a function of the incidence angle of a far IR beam on a film. Using Fresnel equations relating the reflectance and the phase of a beam to the incidence angle and the refractive index, the refractive index of a film is estimated. The refractive index is determined by fitting the experimental reflectance and phase variation curves to the theoretical Fresnel equations.
The method in the ""928 patent uses GHz-THz radiation sources in the far IR region. These sources have wavelengths of 0.1 mm to 1 cm, and therefore the technique is available for materials of a few microns in thickness. The method described in the ""928 patent uses a femtosecond mode-locked laser to excite an emitter, and a sophisticated detection mechanism.
The need remains for a cost-effective technique that provides for the simultaneous measurement of the refractive index and thickness of various film materials.
The present invention provides a method for the simultaneous determination of sample thickness L and index of refraction n. The method comprises forming a sample with first and second surfaces. A radiation beam is also formed and the radiation beam impinges onto the sample at an incidence angle (A1) relative to the perpendicular axis to the first surface. The method also includes reflecting the impinged radiation beam from the first and second surfaces of the sample to form first and second reflected radiation beams. The first and second reflected radiation beams then impinge on a detection device which allows the measurement of the distance (d1) on the detector between the impingement point of the first reflected beam and the impingement point of the second reflected radiation beam. The method further includes altering the first incidence angle to a second incidence angle (A2) and measuring the distance (d2) between the impingement point of a third reflected beam and the impingement point of a fourth reflected beam on the detection device. The method finally provides for obtaining the sample thickness L and sample index of refraction n from the following equations:
d1=[2.L/n].[sin A1/(1xe2x88x92(sin2A1)/n2)1/2] and 
d2=[2.L/n].[sin A2/(1xe2x88x92(sin2A2)/n2)1/2]
Another method for the simultaneous determination of a sample thickness L and index of refraction n, is also provided which involves transmitting the radiation beam through the sample, and intercepting the transmitted radiation beam by the detection device and measuring the distance (d1) between the point on the detection device where the axis intercepts the detection device and the point on the detection device where the transmitted beam impinges on the detection device. This method also includes directing the radiation beam along a second axis with the sample, and transmitting the radiation beam through the sample and measuring a second distance (d2) between a point on the detection device where the second axis intercepts the detection device and a point on the detection device where the transmitted beam impinges on the detection device. This method involves solving the following system of equations for transmitted radiation beams:
d1=L[sinA1xe2x88x92(sin2A1÷2(n2xe2x88x92sin2A1)1/2)] and 
xe2x80x83d2=L[sinA2xe2x88x92(sin2A2÷2(n2xe2x88x92sin2A2)1/2)]
to obtain values for L and n.